This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The chromogranins/secretogranins (granins) are a family of regulated secretory proteins widely distributed in dense-core secretory granules (DCGs) of neuroendocrine, endocrine, and neuronal cells, and includes chromogranin A (CgA), chromogranin B (CgB), and secretogranin II (SgII). Granins function as pro-hormones, giving rise by proteolytic processing to biologically active peptide fragments. Recent evidence suggests that granins may be required for the biogenesis of DCGs, and hormone sequestration in neuroendocrine cells. Using a series of novel granin chimeric photoproteins, we aim to provide a molecular definition of the cellular role of granins in hormone and neurotransmitter storage and release within the sympathetic nervous system, and the adrenal medulla. This project aims to characterize the contribution of CgA to the initiation and formation of DCGs in sympathoadrenal chromaffin cells, and to identify putative granulogenic determinant(s) within the primary structure of this granin. We previously reported that information necessary for CgA trafficking within the regulated pathway of chromaffin cells is contained in the amino-terminal region (CgA1-115) of the granin1. More recently, we have shown that a V-ATPase-mediated acidification of the TGN and/or the immature chromaffin granule contributes to the sorting of CgA to the regulated secretory pathway2 and to the biogenesis of DCGs by a mechanism that may recruit a sorting/granulogenic determinant located within CgA1-115, but not the C-terminal region of the protein 2, 3 We will test the hypothesis that CgA may rescue the formation of secretory granules in secretory organelle-deficient chromaffin cells, and a particular motif (cis determinant) within the primary structure of CgA is essential for such granulogenic role. We propose to express a series of CgA domains fused to EGFP or pHluorin in secretory organelle-deficient chromaffin cells, to investigate and characterize granulogenic cis determinant(s) within the primary structure of CgA. Responses to be investigated include secretagogue-stimulated exocytosis of transiently transfected CgA chimera by TIRF microscopy. The studies will be initially conducted in the sympathoadrenal cell variant A35C, which lacks DCGs and a regulated secretory pathway, and does not express several membrane or soluble secretory granule proteins, including CgA, CgB, and SgII 4. We will extend these studies to primary culture of chromaffin cells isolated from CgA-/- KO mice, which show reduced DCGs number.